Electrical control system



June 21, 1949; KQJ. STIEZFEL. 2,473,617

ELECTRICAL CONTROL SYSTEM Filed 001;. 27, 1944 2 sheets-sham 1RES/STHNCE WHICH DECIIHSES WIT/1 INCREHSE \UF CURRENT.

DE GEE/15E 5 IWTH INCEE 1952' \OF CURREN T June 21, 1949.

Filed Oct. 27, 1944 K. J. STIEFELL 2,473,617

ELECTRICAL CONTROL SYSTEM 2 Sheets-Sheet2 ex g3; v w h F v 55:

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Patented June 21, 1949- UNITED STATES PATENT OFFICE ELECTRICAL CONTROLSYSTEM Karl J. Stiefel, Waltliam, Mass., assignor to RaytheonManufacturing Company, Newton, Mass., a corporation of DelawareApplication October 27, 1944, Serial No. 560,650

subjected to large changes in the ambient temperature. When all of theseconditions are present, such systems become highly unreliable.

In my copending application, Serial No. 556,446,

filed September 29, 1944, there is disclosed a system which overcomesthese defects of the prior .art so that, when the relay is subjected toshock or vibration, the same will not be snapped into ditions involvingshock and vibration or when invention will be made fully apparent tothose.

skilled in the art from a consideration of the following detaileddescription, taken in conjunction with the accompanying drawings inwhich:

Fig. 1 is a circuit diagram of a system embodyin my invention;

Fig. 2 is a circuit diagram of a further embodiment of the invention;and v Fig. 3 is a curve illustrating certain operating characteristicsof the invention.

Referring to the drawings, the solenoid 2 of a relay I is supplied withcurrent from a bridge rectifier 3. The rectifier 3 is supplied withalternating current from a circuit connected across the terminals 4 and5 of a suitable source of supply. In this circuit and in series with thebridge rectifier circuit are the coils 6 and I of a saturable reactor 8.Also connected across the terminals 4 and 5 through a rectifier 9 is abridge circuit, in-

eluding resistors III, II, I2 and I3, one in each of the arms of thebridge. The resistors I0 and II are jointly connected to one of theterminals 4 and 5 and the resistors I2 and I3 are jointly connected tothe other of these terminals. The junction between the resistors III andI2 is connected to a coil I4 of the saturable reactor 8, and thejunction between the resistors II and I3 is connected to the oppositeend of the coil I4. A blocking recti- -fier I5 is included in theconnections between the bridge circuit and the coil I I.

The resistor It may be of the usual type in which the resistance remainssubstantially constant with variations in the current therethrough, orit may be of the type in which the resistance increases with an increasein current. For example, it may be the tungsten filament of a lamp ,or anumber of them. The resistors II and I2 are of a material the resistanceof which rapidly decreases as the current therethrough is increased.Such resistance elements are known per se in the art and may be composedof a ceramic resistance material comprising graphite and siliconcarbide, for example, Thyrite. In such materials the resistancedecreases in accordance with an exponential function of the currentforced through it. It will'be understood that any resistor, of the typein which the resistance decreases rapidly as the current therethroughincreases, is suitable for the purposes of the present invention. Sincethe curve of the resistance-current characteristics of such an elementhas a negative slope, resistance elements of this type are commonly saidto have negative resistance-current characteristics. Also where theresistance is a non-linear function of the current, the resistanceelement may be said to have negative non-linear resistance-currentcharacteristics. The resistor I3 comprises a variable resistance It anda temperature compensating resistance element IT. The element I1 ispreferably the same material as the resistors I I and I2, but isheat-insulated to compensate for the variation of the resistance of theother elements with changes in ambient temperature.

In the operation of the circuit just described, when the alternatingpotential, indicated as Eat, across the supply source is at a normalpredetermined value, say volts, the resistance of the elements I0, I I,I2 and I3 is such that the bridge circuit is balanced and no currentflows through the coil It of the saturable reactor 8. In other words, ifR1, R2, R3 and R4 denote the values of the resistance of the elementsII), I I, I2 and I3, respectively, these values are so chosen that therelation therebetween at the predetermined value of the appliedpotential may be expressed by the equation:

- Bah R R If the potential at the input source falls below ,thispredetermined value, then the value of the the bridge circuit isunbalanced in such a direction under these conditions that no currentcan flow through the coil ll due to the blocking rectifier It. When nocurrent flows through the coil It, the inductive reactance of thesaturable reactor 8 is high and only a low current can flow through thecoils 8 and I and hence the rectified current which is supplied to thesolenoid 2 of the relay l is far below the value required to actuate therelay I. When the potential End of the source increases above thepredetermined value, the resistance of the elements H and i2 is greatlydecreased, the bridge circuit becomes unbalanced, and a current Idcflows through the coil It in such a. direction that it is not blocked bythe rectifier [5. This current through the coil [4 tends to saturate thesaturable reactor 8, a tendency which is increased by the increase incurrent through the coils 6 and 1 due to the increase in the potentialof the source. As the reactor 8 becomes saturated, the impedance to thefiow of current through the coils 6 and I is sharply decreased, andhence the rectified current supplied to the solenoid 2 of the relay I issharply increased to 1 value at which the relay is actuated.

From the foregoing it will be seen that the voltage across the solenoid2 increases far more rapidly than the corresponding increase in the linevoltage as the predetermined cut-off voltage is approached. Therelationship between the voltage across the relay and the line voltagein one successful application of the invention is shown by the curve aof Fig. 3, in which the ordinates represent the voltage across therelay, which in this case has a resistance of 1000 ohms, and theabscissas represent the line or input voltage. In this case the desiredcut-off value of the input voltage was 115 volts. It will be seen thatas the line voltage increases to this value from 110 volts, the voltageacross the relay increases from about 45 to nearly 60 volts. When thesystem of the present invention is not used and the relay is connectedacross the line through the usual current-limiting resistance, thevoltage across the relay varies linearly with variations in the linevoltage, the relation therebetween being that of the curve b. It will beseen that without the system of the present invention, the approach tothe desired cut-off value is much more gradual so that, even though theline voltage is substantially below the predetermined cut-off value ofabout 115 volts, the voltage across the relay is almost sufiicient toactuate the same. Under these conditions, factors independent of theline voltage, such as slight shocks or vibrations to which the relay maybe subjected, may cause the relay to be jolted into open position. Also,small differences in the force required to actuate the relay arising inthe mechanical parts of the same may cause'it to be actuated over asubstantial range of line voltages instead of within a relativelylimited range of voltages close to a desired predeterminedvoltage. Forexample, if any part of the switch mechanism tends to stick slightly,the voltage across the line must be materially above the desired cut-01fvoltage before the switch is opened. In the case of the presentinvention, if the mechanical parts of the switch tend to stick slightly,then a very small increase in the line voltage over the predeterminedcut-oil? value results in a large increase in the voltage across therelay, so that the same is nevertheless actuated,

while the line voltage is still close to the desired cut-off value.

While, in the embodiment of the invention 4 shown in Fig. 1, the relayis supplied with a direct current from a suitable rectifier 3 in serieswith the coils 6 and I of the saturable reactor 8, it will be understoodthat when the relay is of the type suitable for operation on alternatingcurrent, the solenoid 2 of the relay I may be directly connected in thecircuit without the interposition of such a rectifier. Also, in the formshown in Fig. 1, the relay circuit is connected directly across theinput terminals 4 and 5. In some instances it is desirable to provide atransformer between the supply line and the relay, as shown in Fig. 2.Referring to this figure, in which corresponding elements of the systemof Fig. 1 are similarly numbered, current is supplied to the relaycircuit from a transformer 20 having a primary winding 2| connectedacross the input terminals 4 and 5. and a secondary winding 22. Thenumber of turns of the secondary winding may bear any desired step-uprelation to the number of turns of the primary winding. In this figurethe relay l is directly connected in the circuit, and it will beunderstood that it might also be connected through a rectifier in themanner shown in Fig. 1.

While there have been herein described exemplary embodiments of theinvention, other embodiments within the scope of. the appended claimswill be apparent to those skilled in the art from a consideration of theembodiments shown and the teachings hereof. Accordingly a broadinterpretation of the appended claims commensurate with the scope of theinvention within the art is desired.

What is claimed is:

1. In combination, a saturable reactor having a saturable magnetic core,a main winding on said core and adapted to be connected to a source ofalternating current, a control winding on said core, a bridge circuitfor supplying saturating direct current to said control winding inresponse to a predetermined condition, said bridge circuit having fourarms each including a resistor, two of said resistors in diametricallyopposite arms having negative resistance current characteristics,another of said resistors having a substantially constant resistancecurrent characteristic, and the fourth resistor having a small componentof negative resistance current characteristic and a large componenthaving substantially constant resistance current characteristic, saidresistors being arranged to unbalance said bridge circuit when thecurrent therethrough differs from a predetermined value, and anelectromagnetic relay having an operating coil connected in series withsaid main winding, whereby said relay is operated in response to theflow of saturating direct current through said control winding.

2. In combination, a saturable reactor having a saturable magnetic core,a main winding on said core and adapted to be connected to a source ofalternating current, a control winding on said core, a bridge circuitfor supplying saturating direct current to said control winding inresponse to a predetermined condition, said bridge circuit having fourarms each including a resistor, two of said resistors having negativeresistance current characteristics, another of said resistors having asubstantially constant resistance current characteristic, and the fourthresistor having a small component having negative resistance currentcharacteristic, and a large component having substantially constantresistance current characteristic, said resistors being arranged tounbalance said bridge circuit when the current therethrough differs froma predetermined value,

REFERENCES CITED The following. references are of record in-the file ofthis patent:

6 11mm sures ram'rs Number Name Date Fortesque Dec. 26, 1916 BteimneaAug. 20, 1916 Thomas June 4, 1929 Ollendori! et al. 1--- Aug. 16, 1932Gllson Nov. 1, 1932 Nickle May 9, 1933 Fitz Gerald Feb. 20, 1934 LoganJan. 5, 1937 Merrill June 8, 1937 Johnson Mar. 22, 1938 Croflord June 3,1941 McWhirter et al. June 5, 1945 Btiefel Apr. 30, 1946

